The invention relates in general to a method for a continuous measurement of damping of a light beam in an elongated light wave conductor (LWC)-sensor at which only one of its two ends is accessible.
In LWC-sensors for measuring mechanical stresses such as pull tension, bending stresses and torque, the change in the damping of the light passing through the sensor is usually employed as a criterium for the detection of the change of the stress to be detected. According to prior art damping measurements the beginning and the end of the sensors are provided respectively with an optical transmitter and an optical receiver in order to enable a continuous measurement.
For example, in the German Patent DE-PS No. 3,505,234 LWC-pull tension sensor is described in which the light wave conductor (LWC) is embedded in a pull tension resistant wire of fiber reinforced resinous material whereby a non-homogenous intermediate layer is provided between the wire material and the light wave conductor. Presently, the preferred embodiment of the intermediate layer is in the form of a coil of a steel wire wound around the light wave conductor. This known LWC-pull tension sensor is provided at its both ends with connectors for a light passage testing apparatus or a light damping measurement apparatus both including a light transmitter and a light receiver. Since during the elongation of the pull tensioned wire its cross-section decreases, there occur transverse force components which are transferred with an amplified effect against the light wave conductor and produce therein microbendings. The microbendings cause an increase in the light damping which is measured and employed as a measure for the elongation. In this manner, such LWC-tension sensors are fixed in the load direction for example to a part of a prestressed concrete bridge to monitor it for ruptures. However, this known measuring method is unsuitable when only one of the ends of the LWC-sensor is accessible. In this case the damping measurement can be carried out by means of a commercially available back scatter measuring apparatus which is attached to the accessible end of the sensor only and which additionally enables the location of a defect. However, contemporary back scatter concept is based on the transmission of pulses and is unsuitable for a continuous damping measurement and for a continuous monitoring by means of LWC-sensors.